Conventional negative feedback operational amplifier for driving a load unit, such as switched capacitor amplifiers, had a disadvantage of varying the capacitance value of the load unit connected on the output side of the operational amplifier due to, for example, temperature change or replacement of a load element in the load unit and change in a feedback gain, and thus making the system unstable. They were designed to have large circuit margin in consideration of a worst case condition for securing the system stability, resulting in creating other problems, such as sacrificing circuit acceleration and increased power consumption.
As an example of the remedies, a technique is known, as described in the Patent Document 1, that controls phase margin of an operational amplifier by newly providing a phase control circuit having a capacitor and a resistor for phase compensation between a first operational amplification stage and a second operational amplification stage in the operational amplifier to enable to modify a capacitance value of the capacitor for phase compensation and a resistance value of the resistor for phase compensation from outside. This technique secures optimized phase margin by controlling the phase margin to prevent ringing and oscillation of output from an operational amplifier, and thus realizes acceleration by enhancing the properties of the operational amplifier itself.
As another example, a conventional technique is known, as described in the Patent Document 2, that realizes acceleration by newly providing a resistor element, equivalent to a load element in a load unit driven by an operational amplifier, in the load unit and controlling the resistance value of the resistor element in a case that the replacement of the load element causes the capacitance value of the load unit to vary. This technique controls the resistance value of the resistor element to make a time constant of a feedback stage subject to negative feedback to the operational amplifier equivalent to a time constant of the load unit driven by the operational amplifier for optimizing response speed of the operational amplifier and enabling the operational amplifier to develop the properties, and thus realizes acceleration.
[Patent Document 1] JP2004-120564 A
[Patent Document 2] JP2002-190721 A